Python实现快速排序算法

fmwg 5年前

Python 算法 快速排序

# -*- coding: utf-8 -*-      from random import randint, shuffle      def _partition(seq, p, r):      """数组划分,伪码如下:      PARTITION(A, p, r)      1  x ← A[r] // 作为划分主元      2  i ← p-1      3  for j ← p to r-1      4    do if A[j] <= x      5         then i ← i + 1 // 前划分区域的索引      6              exchange A[i] ↔A[j] // 小值交换到前面      7 exchange A[i+1] ↔A[r] // A[r]交换到前区域结尾      8 return i + 1          T(n) = θ(n)      """      x = seq[r]      i = p - 1      for j in range(p, r):          if seq[j] <= x:              i += 1              seq[i], seq[j] = seq[j], seq[i]      i += 1      seq[i], seq[r] = seq[r], seq[i]      return i      def _quick_sort(seq, p, r):      """递归调用,伪码如下:      QUICKSORT(A, p, r)      1  if p < r      2    then q ← PARTITION(A, p, r)      3         QUICKSORT(A, p, q-1)      4         QUICKSORT(A, q+1, r)          T(n) = θ(n^2)      """      if p >= r:          return      q = _partition(seq, p, r)      _quick_sort(seq, p, q - 1)      _quick_sort(seq, q + 1, r)      def quick_sort(seq):      """快速排序      Args:          seq (Sequence): 一个序列对象。      """      _quick_sort(seq, 0, len(seq) - 1)          def _rand_partition(seq, p, r):      """随机取样交换后再划分,伪码如下:      RANDOMIZED-PARTITION(A, p, r)      1  i ← RANDOM(p, r)  // 从A[p..r]中随机选出一个      2  exchange A[r] ↔ A[i] // A[r]与其交换      3  return PARTITION(A, p, r)          T(n) = O(n)      """      i = randint(p, r)      seq[r], seq[i] = seq[i], seq[r]      return _partition(seq, p, r)      def _rand_qsort(seq, p, r):      """随机取样划分方式的递归调用,伪码如下:      RANDOMIZED-QUICKSORT(A, p, r)      1  if p < r      2    then q ← RANDOMIZED-PARTITION(A, p, r)      3         RANDOMIZED-QUICKSORT(A, p, q-1)      4         RANDOMIZED-QUICKSORT(A, q+1, r)          T(n) = O(n^2)      """      if p >= r:          return      q = _rand_partition(seq, p, r)      _rand_qsort(seq, p, q - 1)      _rand_qsort(seq, q + 1, r)      def rand_qsort(seq):      """快速排序(随机化版本)"""      _rand_qsort(seq, 0, len(seq) - 1)          def qsort(L):      """快速排序(简易版本)      更多: Python Cookbook 2nd Edition 第5.11章节      """      if not L: return []      return qsort([x for x in L[1:] if x < L[0]]) + L[0:1] + \             qsort([x for x in L[1:] if x >= L[0]])      if __name__ == '__main__':      import timeit          items = range(10000)      shuffle(items)          def test_sorted():          print(items)          sorted_items = sorted(items)          print(sorted_items)          def test_quick_sort():          print(items)          quick_sort(items)          print(items)          test_methods = [test_sorted, test_quick_sort] # test_rand_qsort, test_qsort      for test in test_methods:          name = test.__name__ # test.func_name          t = timeit.Timer(name + '()', 'from __main__ import ' + name)          print(name + ' takes time : %f' % t.timeit(1))